Method for preparing graphene by liquid-phase ball milling exfoliation

ABSTRACT

Provided is a method for preparing graphene by liquid-phase ball milling exfoliation, including following steps: mixing a transition metal halide salt, a nitrogen source substance and an organic solvent to prepare an intercalation agent; mixing the intercalation agent with graphite, carrying out ball milling, and then performing centrifugation to obtain a graphite intercalation compound; washing and filtering the graphite intercalation compound obtained, adding an expansion agent, and carrying out ultrasonic agitation to obtain a graphene dispersion; and washing, filtering and drying the graphene dispersion to obtain graphene powder.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Chinese Application No. CN201810738621.8 having a filing date of Jul. 6, 2018, the entire contentsof which are hereby incorporated by reference.

FIELD OF TECHNOLOGY

The following relates to the field of graphene preparing technology, andin particular relates to a method for preparing graphene by liquid-phaseball milling exfoliation.

BACKGROUND

Graphene has excellent properties such as high electron mobility,ultrahigh thermal conductivity, good mechanical properties, andremarkable room-temperature Hall effect, which enables graphite andgraphene related materials to be widely used in battery electrodematerials, semiconductor devices, transparent displays, sea-waterdesalination, hydrogen storage materials, aerospace, compositematerials, etc. In view of the excellent properties of graphenematerials and their potential application value, the research andapplication development of graphene continues to heat up at home andabroad. Researchers focus on trying different methods in differentfields to prepare high-quality graphene materials on a large scale, andreducing graphene preparation costs by continuously optimizing andimproving the graphene preparation process, so that their excellentproperties can be applied more widely.

Recently, some progress has been made on how to prepare high-qualitygraphene materials, but how to achieve mass production of high-yield andhigh-quality graphene is still a difficulty in research. Ball milling isa mechanical exfoliation method. The preparation of graphene by ballmilling has attracted widespread attention from researchers due to itssimple production process, low production cost, high yield and goodquality.

The known art discloses a graphene preparation method, in whichintercalation is carried out by using a composite intercalation agentfirst, and then exfoliation is achieved by ball milling shear to preparegraphene. In the preparation technology, a surfactant with a highboiling point is used in the preparation process of the compositeintercalation agent, which is unfavorable for the subsequent dryingtreatment and thus affects the graphene quality; moreover, as theultrasonic intercalation is carried out before ball milling, thepreparation process is complicated, and the long-time ultrasonicationnot only affects the intercalation, but also causes the intercalationagent to fall off from the space between the layers, which is notconducive to the exfoliation of graphene, resulting in a low yield andlow quality of the prepared graphene.

The known art also discloses a method for preparing graphene, in whichintercalation is also carried out before ball milling, and an acidhaving strong oxidizing property and another oxidizing agent are used asan intercalation agent, and an expansion agent used therein is also anoxidizing substance. This method is essentially no different from thepreparation of graphene by redox, does not reduce the defect degree ofgraphene, but also is more complicated than the redox preparationmethod.

According to the above description, the preparation of graphene by ballmilling at present mainly adopts a preparation scheme ofpre-intercalation and then ball milling, and generally uses a surfactantand enhancer and other experimental conditions that affect or destroythe graphene quality, in the preparation process, resulting in a lowyield and low quality of the prepared graphene.

SUMMARY

An aspect relates to a method for preparing graphene by liquid-phaseball milling exfoliation, by means of which high-quality and high-yieldgraphene can be obtained, and the method is energy-saving andenvironmentally friendly.

The aspect is achieved by the technical solution: a method for preparinggraphene by liquid-phase ball milling exfoliation in embodiments of thepresent invention, including the following steps:

-   -   S1: mixing a transition metal halide salt, a nitrogen source        substance and an organic solvent to prepare an intercalation        agent;    -   S2: mixing the intercalation agent obtained in the step S1 with        graphite, carrying out ball milling, and then performing        centrifugation to obtain a graphite intercalation compound;    -   S3: washing and filtering the graphite intercalation compound        obtained in the step S2, adding an expansion agent, and carrying        out ultrasonic agitation to obtain a graphene dispersion; and    -   S4: washing, filtering and drying the graphene dispersion        obtained in the step S3 to obtain graphene powder.

Compared with the known art, embodiments of the present invention havethe following beneficial effects:

-   -   (1) The intercalation agent is prepared by using the synergistic        effect among the transition metal halide salt, the nitrogen        source substance and the organic solvent, wherein the transition        metal halide salt can form a eutectic with the nitrogen source        substance or the organic solvent, and the melting point thereof        is lower than that of each component, and the mixed        intercalation agent is even liquid at room temperature, and is        inserted into graphite to form the graphite intercalation        compound, thereby lowering the reaction temperature, and the        preparation cost and difficulty; and a hydrogen bond can also be        formed between the nitrogen source substance and the organic        solvent, so that the bonding structure of the nitrogen source        substance and the organic solvent is present stably between the        graphene layers, thereby avoiding interlayer stacking of the        prepared graphene, thus improving the exfoliation efficiency and        the product quality.    -   (2) In the subsequent expansion treatment, under the ultrasonic        effect, the expansion agent moves to the space between the        graphite layers and decomposes to generate a gas, so that the        interlayer spacing of graphite is further increased, which is        beneficial to further exfoliation of graphite, thereby greatly        improving the yield of graphene.    -   (3) The intercalation agent does not undergo a chemical reaction        during the ball milling process, and the intercalation agent and        the graphite intercalation compound can be separated by        centrifugation, and the separated intercalation agent can be        recycled, which is energy-saving and environmentally friendly.

Further, in the step S1, the mass ratio of the transition metal halidesalt, the nitrogen source substance and the organic solvent is(1-10):1:(2-10).

Further, the transition metal halide salt is any one or more ofmanganese chloride, chromium chloride, copper chloride, nickel chloride,ferrous bromide, ferric bromide, ferric chloride, and ferric chloridehexahydrate. As an electron-accepting intercalator, the transition metalhalide salt accepts π electrons between the graphite layers during theintercalation process, and becomes negative ions and enters the spacebetween the graphite layers.

Further, the nitrogen source substance is any one or more of urea,dicyandiamide and melamine. Under the action of nitrogen source such asurea, the edges of graphite can be doped with nitrogen to formfunctional groups such as pyrrole and pyridine, so that the edges of thegraphite layers turn up, thereby facilitating inserting theintercalation agent and the expansion agent between the layers toachieve interlayer exfoliation of graphite.

Further, the organic solvent is any one or more of ethanol, ethyleneglycol, isopropanol, 1,2-propanediol, glycerol, formic acid, aceticacid, methyl acetate, ethyl acetate and ethyl formate. The organicsolvent is added to achieve a buffering effect in the ball millingprocess, thereby avoiding damage to the graphene structure by thesolid-phase ball milling; moreover, it can disperse and stabilize thegraphene obtained by exfoliation to prevent re-stacking of the graphene.

Further, in the step S2, the mass ratio of the intercalation agent tothe graphite is (40-200):1.

Further, in the step S2, zirconia balls are used in the ball millingprocess, and the volume ratio of the intercalation agent to the zirconiaballs is (1-3): 1, and the total volume of the intercalation agent andthe zirconia balls accounts for 25-60% of the volume of the jar mill.The ball milling speed is 200-700 rpm, and the ball milling time is 2-48hours. By setting the conditions, it is ensured that the intercalationagent can completely submerge the zirconia balls, so that the zirconiaballs can fully exert the shearing action between the balls; while fullyshearing, the liquid-phase ball milling can increase the bufferingeffect of the balls and graphite flakes, and reduce the defects of theproduced graphene layers, thereby obtaining a high-yield, large-flakegraphene material.

Further, in the step S2, the graphite is any one or more of expandedgraphite, expandable graphite, natural flake graphite and graphitepowder.

Further, in the step S3, the expansion agent is any one or more ofhydrogen peroxide, sodium borohydride and ammonium bicarbonate. Theexpansion agent decomposes between the layers to generate a gas, whichdestroys the van der Waals force between the graphite layers, so thatthe interlayer spacing of graphite is further increased, therebyexfoliating graphene.

Further, in the step S4, the drying is carried out at a temperature of60-80° C. for 12-24 hours.

For the sake of better understanding and implementation, embodiments ofthe present invention are described in detail below in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION

Some of the embodiments will be described in detail, with reference tothe following figures, wherein like designations denote like members,wherein:

FIG. 1 illustrates ultraviolet absorbances of graphene dispersions ofEmbodiments 1 to 7 and a comparative single-layer graphene dispersion;

FIG. 2 shows a scanning electron microscope image of expanded graphitein Embodiment 1; and

FIG. 3 shows a scanning electron microscope image of a graphene productin Embodiment 1.

DETAILED DESCRIPTION

In view of the problems in the preparation of graphene by the ballmilling method at present that defects are serious and an organicsolvent with a high boiling point is difficult to remove, in embodimentsof the present invention, the intercalation agent is changed to atransition metal halide salt and a nitrogen source substance withoutstrong oxidizing property and an organic solvent that is easy to remove,which are used in combination as an environmentally friendlyintercalation agent, which has an intercalating effect during ballmilling and achieves partial exfoliation, so that the complicatedprocess of ball milling after intercalation can be avoided; then simplecleaning is carried out to remove the intercalation agent on the surfaceof the intercalation compound; after filtering, the intercalationcompound is further subjected to expansion treatment to obtainhigh-quality and high-yield graphene. The operation is simple and theproduction cost is low, which are favorable for promoting the massproduction of graphene. Moreover, the corrosion effect of a strongoxidant on ajar mill is avoided, thereby achieving the combination ofthe intercalation and the ball-milling exfoliation process, which notonly improves the exfoliation efficiency, but also can maintain theintegrity of the graphene crystal structure, thus expanding theapplications of graphene in the fields of energy storage materials,biomaterials and the like.

The method for preparing graphene by liquid-phase ball millingexfoliation in embodiments of the present invention includes thefollowing the steps:

-   -   S1: mixing a transition metal halide salt, a nitrogen source        substance and an organic solvent to prepare an intercalation        agent;    -   S2: mixing the intercalation agent obtained in the step S1 with        graphite, carrying out ball milling, and then performing        centrifugation to obtain a graphite intercalation compound;    -   S3: washing and filtering the graphite intercalation compound        obtained in the step S2, adding an expansion agent, and carrying        out ultrasonic agitation to obtain a graphene dispersion; and    -   S4: washing, filtering and drying the graphene dispersion        obtained in the step S3 to obtain graphene powder.

Specifically, in the step S1, the mass ratio of the transition metalhalide salt, the nitrogen source substance and the organic solvent is(1-10):1:(2-10). The transition metal halide salt is any one or more ofmanganese chloride, chromium chloride, copper chloride, nickel chloride,ferrous bromide, ferric bromide, ferric chloride, and ferric chloridehexahydrate. The nitrogen source substance is any one or more of urea,dicyandiamide and melamine. The organic solvent is any one or more ofethanol, ethylene glycol, isopropanol, 1,2-propanediol, glycerol, formicacid, acetic acid, methyl acetate, ethyl acetate and ethyl formate.

In the step S2, the mass ratio of the intercalation agent to thegraphite is (40-200):1. Zirconia balls are used in the ball millingprocess, and the volume ratio of the intercalation agent to the zirconiaballs is (1-3): 1, and the total volume of the intercalation agent andthe zirconia balls accounts for 25-60% of the volume of ajar mill. Theball milling speed is 200-700 rpm, and the ball milling time is 2-48hours. The speed during centrifugation is 8000-10000 rpm. The graphiteis any one or more of expanded graphite, expandable graphite, naturalflake graphite and graphite powder.

In the step S3, the expansion agent is any one or more of hydrogenperoxide, sodium borohydride and ammonium bicarbonate. The agitatingspeed is 200-600 r/min.

In the step S4, the drying process is carried out by using a vacuum ovenfor drying at a temperature of 60-80° C. for 12-24 hours.

Further description is provided below in conjunction with specificembodiments.

Embodiment 1

1 g of expanded graphite was poured into a dry jar mill, then 10 g offerric chloride, 10 g of urea, 20 g of isopropanol were mixed uniformlyand poured into the jar mill so that zirconia beads were submergedtherein; the jar mill was fixed symmetrically to a planetary ball mill,the rotating speed was adjusted to 700 rpm, and ball milling was carriedout continuously for 48 hours; after the ball milling, the graphiteintercalation compound was separated from the intercalation agent byhigh-speed centrifugation at 10,000 rpm, and then the intercalationcompound was simply washed with water to remove the intercalation agenton the surface of the graphite intercalation compound; after filtration,a filter cake was placed into a flask, 50 ml of 5% hydrogen peroxide wasadded therein, water-bath sonication was carried out for 20 min, and thesolution was stirred for 2 h to decompose hydrogen peroxide; then themixture was filtered, washed 3 times, and dried at 80° C. for 12 h toobtain graphene powder.

0.06 g of the obtained graphene powder was dispersed in a 50 mg/mlaqueous urea solution to prepare a graphene dispersion with aconcentration of 0.024 mg/ml. Then, moderate sonication was carried outfor 15 min, and the absorbance of the graphene dispersion at 270 nm wasmeasured by ultraviolet-visible spectroscopy and compared with theabsorbance of a prepared single-layer graphene dispersion with the sameconcentration at 270 nm to measure the concentration of graphene andcalculate the yield of graphene.

The single-layer graphene for comparison was purchased from NanjingXFNANO Materials Tech Co., Ltd, and the model of the graphene wasXF001W.

Embodiment 2

1 g of 300-mesh flake graphite was poured into a dry jar mill, then 5 gof copper chloride, 5 g of urea, 50 g of absolute ethanol were mixeduniformly and poured into the jar mill so that zirconia beads weresubmerged therein. The jar mill was fixed symmetrically to a planetaryball mill, the rotating speed was adjusted to 500 rpm, and ball millingwas carried out continuously for 48 hours; after the ball milling, thegraphite intercalation compound was separated from the intercalationagent by high-speed centrifugation at 10,000 rpm, and then theintercalation compound was simply washed with water to remove theintercalation agent on the surface of the graphite intercalationcompound; after filtration, a filter cake was placed into a flask, 50 mlof 5% sodium borohydride solution was added therein, water bathsonication was carried out for 20 min, then the pH of the solution wasadjusted to make the solution acidic, and stirring was carried out for 2h to decompose sodium borohydride; then the mixture was filtered, washed3 times, and dried at 60° C. for 24 h to obtain graphene powder.

0.06 g of the obtained graphene powder was dispersed in a 50 mg/mlaqueous urea solution to prepare a graphene dispersion with aconcentration of 0.024 mg/ml. Then, moderate sonication was carried outfor 15 min, and the absorbance of the graphene dispersion at 270 nm wasmeasured by ultraviolet-visible spectroscopy and compared with theabsorbance of a prepared single-layer graphene dispersion with the sameconcentration at 270 nm to measure the concentration of graphene andcalculate the yield of graphene.

The single-layer graphene for comparison was purchased from NanjingXFNANO Materials Tech Co., Ltd, and the model of the graphene wasXF001W.

Embodiment 3

1 g of graphite powder was poured into a dry jar mill, then 50 g offerric chloride hexahydrate, 5 g of urea, 10 g of ethylene glycol weremixed uniformly and poured into the jar mill so that zirconia beads weresubmerged therein; the jar mill was fixed symmetrically to a planetaryball mill, the rotating speed was adjusted to 400 rpm, and ball millingwas carried out continuously for 48 hours; after the ball milling, thegraphite intercalation compound was separated from the intercalationagent by high-speed centrifugation at 10,000 rpm, and then theintercalation compound was simply washed with water to remove theintercalation agent on the surface of the graphite intercalationcompound; after filtration, a filter cake was placed into a flask, 50 mlof 5% hydrogen peroxide was added therein, water-bath sonication wascarried out for 20 min, and the solution was stirred for 2 h todecompose hydrogen peroxide; then the mixture was filtered, washed 3times, and dried at 80° C. for 12 h to obtain graphene powder.

0.06 g of the obtained graphene powder was dispersed in a 50 mg/mlaqueous urea solution to prepare a graphene dispersion with aconcentration of 0.024 mg/ml. Then, moderate sonication was carried outfor 15 min, and the absorbance of the graphene dispersion at 270 nm wasmeasured by ultraviolet-visible spectroscopy and compared with theabsorbance of a prepared single-layer graphene dispersion with the sameconcentration at 270 nm to measure the concentration of graphene andcalculate the yield of graphene.

The single-layer graphene for comparison was purchased from NanjingXFNANO Materials Tech Co., Ltd, and the model of the graphene wasXF001W.

Embodiment 4

1 g of graphite powder was poured into a dry jar mill, then 20 g ofchromium chloride, 5 g of melamine, 20 g of ethylene glycol, and 10 g ofglycerin were mixed uniformly and poured into the jar mill so thatzirconia beads were submerged therein; the jar mill was fixedsymmetrically to a planetary ball mill, the rotating speed was adjustedto 600 rpm, and ball milling was carried out continuously for 48 hours;after the ball milling, the graphite intercalation compound wasseparated from the intercalation agent by high-speed centrifugation at10,000 rpm, and then the intercalation compound was simply washed withhot water to remove the intercalation agent on the surface of thegraphite intercalation compound; after filtration, a filter cake wasplaced into a flask, 50 ml of 5% ammonium bicarbonate was added therein,water-bath sonication was carried out at 75° C. for 20 min, and thesolution was stirred for 2 h; then the mixture was filtered, washed 3times, and dried at 80° C. for 12 h to obtain graphene powder.

0.06 g of the obtained graphene powder was dispersed in a 50 mg/mlaqueous urea solution to prepare a graphene dispersion with aconcentration of 0.024 mg/ml. Then, moderate sonication was carried outfor 15 min, and the absorbance of the graphene dispersion at 270 nm wasmeasured by ultraviolet-visible spectroscopy and compared with theabsorbance of a prepared single-layer graphene dispersion with the sameconcentration at 270 nm to measure the concentration of graphene andcalculate the yield of graphene.

The single-layer graphene for comparison was purchased from NanjingXFNANO Materials Tech Co., Ltd, and the model of the graphene wasXF001W.

Embodiment 5

1 g of expandable graphite was poured into a dry jar mill, then 25 g ofnickel chloride, 5 g of dicyandiamide, 50 g of methyl acetate were mixeduniformly and poured into the jar mill so that zirconia beads weresubmerged therein; the jar mill was fixed symmetrically to a planetaryball mill, the rotating speed was adjusted to 400 rpm, and ball millingwas carried out continuously for 48 hours; after the ball milling, thegraphite intercalation compound was separated from the intercalationagent by high-speed centrifugation at 10,000 rpm, and then theintercalation compound was simply washed with hot water to remove theintercalation agent on the surface of the graphite intercalationcompound; 50 ml of 5% sodium borohydride solution was added therein,water bath sonication was carried out for 20 min, then the pH of thesolution was adjusted to make the solution acidic, and stirring wascarried out for 2 h to decompose sodium borohydride; then the mixturewas filtered, washed 3 times, and dried at 80° C. for 12 h to obtaingraphene powder.

0.06 g of the obtained graphene powder was dispersed in a 50 mg/mlaqueous urea solution to prepare a graphene dispersion with aconcentration of 0.024 mg/ml. Then, moderate sonication was carried outfor 15 min, and the absorbance of the graphene dispersion at 270 nm wasmeasured by ultraviolet-visible spectroscopy and compared with theabsorbance of a prepared single-layer graphene dispersion with the sameconcentration at 270 nm to measure the concentration of graphene andcalculate the yield of graphene.

The single-layer graphene for comparison was purchased from NanjingXFNANO Materials Tech Co., Ltd, and the model of the graphene wasXF001W.

Embodiment 6

1 g of expanded graphite was poured into a dry jar mill, then 10 g offerrous bromide, 10 g of urea, and 50 g of ethyl acetate were mixeduniformly and poured into the jar mill so that zirconia beads weresubmerged therein; the jar mill was fixed symmetrically to a planetaryball mill, the rotating speed was adjusted to 500 rpm, and ball millingwas carried out continuously for 48 hours; after the ball milling, thegraphite intercalation compound was separated from the intercalationagent by high-speed centrifugation at 10,000 rpm, and then theintercalation compound was simply washed with ethanol to remove theintercalation agent on the surface of the graphite intercalationcompound; after filtration, a filter cake was placed into a flask, 50 mlof 5% ammonium bicarbonate was added therein, water-bath sonication wascarried out at 75° C. for 20 min, and the solution was stirred for 2 h;then the mixture was filtered, washed 3 times, and dried at 60° C. for24 h to obtain graphene powder.

0.06 g of the obtained graphene powder was dispersed in a 50 mg/mlaqueous urea solution to prepare a graphene dispersion with aconcentration of 0.024 mg/ml. Then, moderate sonication was carried outfor 15 min, and the absorbance of the graphene dispersion at 270 nm wasmeasured by ultraviolet-visible spectroscopy and compared with theabsorbance of a prepared single-layer graphene dispersion with the sameconcentration at 270 nm to measure the concentration of graphene andcalculate the yield of graphene.

The single-layer graphene for comparison was purchased from NanjingXFNANO Materials Tech Co., Ltd, and the model of the graphene wasXF001W.

Embodiment 7

1 g of 300-mesh flake graphite was poured into a dry jar mill, then 25 gof ferric chloride, 25 g of nickel chloride, 5 g of urea, 50 g ofethanol were mixed uniformly and poured into the jar mill so thatzirconia beads were submerged therein; the jar mill was fixedsymmetrically to a planetary ball mill, the rotating speed was adjustedto 600 rpm, and ball milling was carried out continuously for 48 hours;after the ball milling, the graphite intercalation compound wasseparated from the intercalation agent by high-speed centrifugation at10,000 rpm, and then the intercalation compound was simply washed withwater to remove the intercalation agent on the surface of the graphiteintercalation compound; after filtration, a filter cake was placed intoa flask, 50 ml of 5% hydrogen peroxide was added therein, water-bathsonication was carried out for 20 min, and the solution was stirred for2 h to decompose hydrogen peroxide; then the mixture was filtered,washed 3 times, and dried at 80° C. for 12 h to obtain graphene powder.

0.06 g of the obtained graphene powder was dispersed in a 50 mg/mlaqueous urea solution to prepare a graphene dispersion with aconcentration of 0.024 mg/ml. Then, moderate sonication was carried outfor 15 min, and the absorbance of the graphene dispersion at 270 nm wasmeasured by ultraviolet-visible spectroscopy and compared with theabsorbance of a prepared single-layer graphene dispersion with the sameconcentration at 270 nm to measure the concentration of graphene andcalculate the yield of graphene.

The single-layer graphene for comparison was purchased from NanjingXFNANO Materials Tech Co., Ltd, and the model of the graphene wasXF001W.

The absorbance of the graphene dispersions of Embodiments 1 to 7 and theabsorbance of the single-layer graphene dispersion for comparison weredetected, and the results are shown in FIG. 1. The graphene yields ofEmbodiments 1 to 7 were calculated, and the results are shown in Table 1below.

TABLE 1 Summary of reagent types and graphene yields of Embodiments 1-7Embo- Expansion Graphene diment intercalation agent Graphite agent yield1 Ferric chloride, Expanded Hydrogen 52% urea, isopropanol graphiteperoxide 2 Copper Chloride, 300-mesh Sodium 45% urea, absolute flakeborohydride ethanol graphite solution 3 Ferric chloride GraphiteHydrogen 65% hexahydrate, urea, powder peroxide ethylene glycol 4Chromium chloride, Grapliite Ammonium 35% melamine, ethylene powderbicarbonate glycol, glycerin solution 5 Nickel chloride, ExpandableSodium 32% dicyandiamide, graphite borohydride methyl acetate solution 6Ferrous bromide, Expanded Ammonium 40% urea, ethyl acetate graphitebicarbonate solution 7 Ferric chloride, 300-mesh Hydrogen 37% nickelchloride, flake peroxide urea, ethanol graphite

It can be seen from FIG. 1 that the final ball-milled graphene yields inthe different embodiments are different, wherein the yield in Embodiment3 is the highest, which is attributed to the good synergistic effect offerric chloride hexahydrate, urea and ethylene glycol.

In addition, the morphology of the expanded graphite and the grapheneproduct in Embodiment 1 was tested to obtain a scanning electronmicroscope (SEM) image, as shown in FIGS. 2 and 3, wherein at the samemagnification (10 K times), the surface of expanded graphite (as shownin FIG. 2) is smooth and exhibits irregular-size flake structures, whilethe ball-milled graphene (shown in FIG. 3) has flexibility and obviouspleats, with some graphene layers folded and stacked on each other.

Compared with the known art, embodiments of the present invention havethe following beneficial effects:

-   -   (1) The intercalation agent is prepared by using the synergistic        effect among the transition metal halide salt, the nitrogen        source substance and the organic solvent, wherein the transition        metal halide salt can form a eutectic with the nitrogen source        substance or the organic solvent, and the melting point thereof        is lower than that of each component, and the mixed        intercalation agent is even liquid at room temperature, and is        inserted into graphite to form the graphite intercalation        compound, thereby lowering the reaction temperature, and the        preparation cost and difficulty; and a hydrogen bond can also be        formed between the nitrogen source substance and the organic        solvent, so that the bonding structure of the nitrogen source        substance and the organic solvent is present stably between the        graphene layers, thereby avoiding interlayer stacking of the        prepared graphene, thus improving the exfoliation efficiency and        the product quality.    -   (2) In the subsequent expansion treatment, under the ultrasonic        effect, the expansion agent moves to the space between the        graphite layers and decomposes to generate a gas, so that the        interlayer spacing of graphite is further increased, which is        beneficial to further exfoliation of graphite, thereby greatly        improving the yield of graphene.    -   (3) The intercalation agent does not undergo a chemical reaction        during the ball milling process, and the intercalation agent and        the graphite intercalation compound can be separated by        centrifugation, and the separated intercalation agent can be        recycled, which is energy-saving and environmentally friendly.

Although the present invention has been disclosed in the form ofpreferred embodiments and variations thereon, it will be understood thatnumerous additional modifications and variations could be made theretowithout departing from the scope of the invention.

For the sake of clarity, it is to be understood that the use of ‘a’ or‘an’ throughout this application does not exclude a plurality, and‘comprising’ does not exclude other steps or elements.

What is claimed:
 1. A method for preparing graphene by liquid-phase ballmilling exfoliation, comprising following steps: mixing a transitionmetal halide salt, a nitrogen source substance and an organic solvent toprepare an intercalation agent; mixing the intercalation agent obtainedwith graphite, carrying out ball milling, and then performingcentrifugation to obtain a graphite intercalation compound; washing andfiltering the graphite intercalation compound, adding an expansionagent, and carrying out ultrasonic agitation to obtain a graphenedispersion; and washing, filtering and drying the graphene dispersion toobtain graphene powder.
 2. The method of claim 1, wherein the mass ratioof the transition metal halide salt, the nitrogen source substance andthe organic solvent is (1-10):1:(2-10).
 3. The method of claim 1,wherein the transition metal halide salt is any one or more of manganesechloride, chromium chloride, copper chloride, nickel chloride, ferrousbromide, ferric bromide, ferric chloride, and ferric chloridehexahydrate.
 4. The method of claim 2, wherein the transition metalhalide salt is any one or more of manganese chloride, chromium chloride,copper chloride, nickel chloride, ferrous bromide, ferric bromide,ferric chloride, and ferric chloride hexahydrate.
 5. The method of claim3, wherein the nitrogen source substance is any one or more of urea,dicyandiamide and melamine.
 6. The method of claim 4, wherein thenitrogen source substance is any one or more of urea, dicyandiamide andmelamine.
 7. The method of claim 3, wherein the organic solvent is anyone or more of ethanol, ethylene glycol, isopropanol, 1,2-propanediol,glycerol, formic acid, acetic acid, methyl acetate, ethyl acetate andethyl formate.
 8. The method of claim 4, wherein the organic solvent isany one or more of ethanol, ethylene glycol, isopropanol,1,2-propanediol, glycerol, formic acid, acetic acid, methyl acetate,ethyl acetate and ethyl formate.
 9. The method of claim 1, wherein themass ratio of the intercalation agent to the graphite is (40-200):1. 10.The method of claim 9, wherein zirconia balls are used in the ballmilling process, and the volume ratio of the intercalation agent to thezirconia balls is (1-3):1, and the total volume of the intercalationagent and the zirconia balls accounts for 25-60% of the volume of thejar mill, and wherein the ball milling speed is 200-700 rpm, and theball milling time is 2-48 hours.
 11. The method of claim 1, wherein thegraphite is any one or more of expanded graphite, expandable graphite,natural flake graphite and graphite powder.
 12. The method of claim 1,wherein the expansion agent is any one or more of hydrogen peroxide,sodium borohydride and ammonium bicarbonate.
 13. The method of claim 1,wherein in the drying is carried out at a temperature of 60-80° C. for12-24 hours.